1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, and more particularly, it relates to a flash memory.
2. Description of the Related Art
A nonvolatile semiconductor memory such as a NAND flash memory has the advantage of high capacity and nonvolatility, and has recently begun to be used in various electronic devices such as a portable audio device.
In the NAND flash memory, memory cells are increasingly shrunk to provide a greater storage capacity. Along with the shrinking of the memory cells, degradation of characteristics of memory cells that had not matter in conventional element sizes has started to be obvious in shrunk memory cells. This degradation has a great effect on the memory cells and on the whole operation of the flash memory.
For example, in a normal flash memory, memory cells adjacent in the extending direction of a word line share a control gate electrode as the word line and an inter-electrode insulating film. It is known that when the inter-electrode insulating film has a stacked structure, electrons move in the interface between two stacked insulating films (see Jpn. Pat. Appln. KOKAI Publication No. 2001-168306). The movement of this charge varies the threshold voltage (data) of the memory cells adjacent in the extending direction of the word line, which degrades data retention characteristics of the memory cells. The distance between the adjacent memory cells is also reduced because of the shrinking of the memory cells, and the degradation of the data retention characteristics is therefore also obvious.